The present invention is directed to a method for producing polycrystalline semiconductor material layers by vapor phase deposition. More specifically, the present invention is directed to a method wherein a gaseous compound containing the semiconductor material is decomposed in a plasma-induced manner in a reactor and the semiconductor material is deposited on substrates that are located in the reactor, hydrogen being added to the plasma.
An example of such a method, for the manufacture of silicon thin-film transistors, is set forth in Electronic Letters, Vol. 23, No. 6, pages 288/289 (Mar. 12, 1987). As disclosed therein, the deposition proceeds through a plasma-assisted chemical vapor deposition (CVD) onto special glasses. The deposition occurs at temperatures of approximately 580.degree. C. The reaction gas consists of silane (SiH.sub.4) mixed with hydrogen. The size of the crystallites are between 80 and 100 nm. The gases in the plasma are ionized approximately 1%.
In known procedures, high substrate temperatures are required for the deposition of polycrystalline layers. In these procedures, this is true even when the deposition occurs with the assistance of a plasma-assisted CVD. Due to the high temperatures, when it is required that the substrate is transparent, the use of silica glass, or of other special glasses, as a substrate is necessary.
It is known to add hydrogen to the plasma during these procedures. When hydrogen is added to the plasma, the growing layer (that can be composed, for example, of silicon) contains crystallites that are surrounded by amorphous silicon. This amorphous silicon is etched away in selective manner by activated hydrogen in the plasma. When sufficient hydrogen is present, the amorphous material can be etched away as it is created. The silicon crystallites then remain on the substrate. The silicon crystallites continue to grow and form a continuous film. A continuous layer of polycrystalline material can be created in this fashion.
The delivery of gaseous hydrogen to the flow discharge, promotes the formation of silicon crystallites. At low temperatures, however, it is not possible to achieve the formation of a continuous polycrystalline layer. In the plasma, only a small part of the hydrogen is activated. The limited amount of hydrogen so activated, however, is not sufficient to in turn etch the amorphous silicon away that arises.